A support structure for an offshore wind turbine must be stable and safe. Wind turbines can operate satisfactorily when motions due to wind current and waves are relatively small. However large motions of surge, sway, pitch, roll, yaw, and heave will result in inefficient operation or shutdown. Therefore, the support structure must have relatively small motions. The support structure motions are affected by the force of wind on the turbine blades, nacelle, and tower, by the wave and current forces on the submerged structure, by the period of the waves, by the period of the rotating turbine blades, and by the natural period of the entire structure, i.e. the time it takes to complete one cycle of motion when subjected to an excitation force. This period depends on the mass of the structure including the added mass of surrounding water, the stiffness of the structure itself (e.g. tower flexing and blade flexing), and the stiffness of the anchoring system of the structure.
If the period of the support structure is synchronous with the period of the waves, the wave forces will be amplified, leading to disruption of operations and/or structural failure. Therefore it is desirable to ensure that the natural period of the support structure differs substantially from the wave periods for all reasonably expected waves.
Vortex Induced Motions (VIMs) are another factor affecting the motions of a floating wind turbine support structure. VIMs are defined as motions caused by vortexes that form from current moving past a structure.